Hard Disk Drive Parts – Clear and Simplified

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Diagram of a computer hard disk drive © Surachit


To store information your computer or laptop may use a Hard Disk Drive (HDD). A hard disk drive is a non-volatile storage device that is capable of retaining data even if you turn off the computer. It works by storing data magnetically on one or several platters with a very thin magnetic coating organized into many billions of sections. The bottom of the hard disk drive is known as the base casting with the logic board on it and ports near it; a cover on top hides the cover chamber with the mechanical parts within known collectively as the head-disk assembly.

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By © Raimond Spekking / CC BY-SA 4.0 (via Wikimedia Commons), CC BY-SA 4.0


Usually made of aluminium substrate the platters only serve to support the media layer of the hard disk drive. Rigidity, lightweight, stability, and cost-effective are reasons why other materials have not been considered. The platters are coated with a very thin layer (only a few millionths of an inch thick) of magnetic material using methods including magnetron sputtering and electroplating. The platters have some degree of protection from foreign material, accidental contact from the read/write heads, and ‘head crashes’ with a super-thin, lubricating layer.

Aufnahme einzelner Magnetisierungen gespeicherter Bits auf einem Festplatten Platter.
Recording of single magnetisations of bits on a 200MB Harddisk-platter (recording visualized using CMOS-MagView). © By Matesy GmbH

Organization of Platters

Tracks, sectors, and clusters make it possible to store large amounts of data in an organized fashion. Each platter has thousands of concentric circles that are tracks ,akin to the rings of a tree. The tracks break down into sectors, of which there can be thousands within one track, and groups of sectors are named clusters.

HDD read write head
English: Read-write head of a Seagate ST3000DM001 3TB hard disk drive (manufactured in 2013). Showing the side that faces the disk platter. The dark rectangle is a slider which maintains a thin air cushion between itself and the platter. Its dimensions are 1.25mm by 0.78mm. The platter surface moves from right to left. Mosaic of roughly 10 partial images. © By Roman Starkov

Reading and Writing

Reading and writing the data from the platters are the read/write heads. These are small magnets connected to the read/write arms with each platter having one above and one below positioned 3 to 20 millionths of inch above the platter surface. As the platter spins the arm moves towards the centre and out to the edge allowing the read/write heads to access the entire surface of the platter. The heads convert the stored data into magnetic pulses and vice-versa for reading.

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Head stack of a Western Digital hard disk (WD2500JS-00MHB0) © By Suit

The Arms

The arm motor, also known as the actuator, moves the arm through the tray and positions and determines how fast the arm is able to move different parts of the platter. Modern HDDs use voice coils for better positioning and accuracy, faster operation, and reliability over the predecessor which were stepper motors.

Using a voice coil the actuator moves the arm of the HDD to perform read or write operations. The logic behind loudspeakers – a coil inside a magnetic field via a magnet – allows the actuator to move.

All of these parts together are known as the read/write assembly or read/write stack.

The read/write arms of a hard disk drive hold the read/write heads and move them about the platter surface. Triangular shapes provide stability and reduced weight. The head is connected via a head slider – this, together with the actuator, forms one unit.

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A dismantled 10GB Quantum Fireball hard drive © By ed g2s

Spinning The Platters

What makes the platters rotate at the correct speed and keep them in the correct position is the spindle motor. Stable, reliable, and consistent turning power for long periods of time are the signs of a good spindle motor, with speeds of 5,400RPM, 7200RPM, 10000RPM, and higher capable depending on the hard disk drive.

1280px Toshiba MK1403MAV hard disk controller 91551
By © Raimond Spekking / CC BY-SA 4.0 (via Wikimedia Commons), CC BY-SA 4.0

The Logic Board

The Printed Circuit Board (PCB), or logic board, located on the underside of the hard disk drive controls the reading and writing processes as well as the data coming to and from the hard disk drive. A flexible connector allows movement of data from the logic board to the read/write heads and the platters; ports allow data transfer to a computer’s motherboard or other PCB.

Circuitry on the logic board provides the functionality needed to control the hard disk drive and other functions. The largest circuit controls data to and from the hard disk drive, hard disk drive motor control, requesting read and write operations, and other.

You may find a flash-ROM circuit with the hard disk drive firmware on the logic board; the controller uses the program that is inside the firmware.

The Random Access Memory (RAM) chip determines the rate at which data is transferred – the higher the capacity the better the performance. To find out, divide the value of the chip by eight. For example, a 64 Megabit (Mb) chip would equate to 8MB of hard disk drive buffer (Mb is used for memory chip capacity and MB is memory capacity).

Two separate connectors allow power and data to be transferred to and from the computer. The data connector is also known as an interface that come in either SATA (Serial ATA) or ATA (Advanced Technology Attachment) types, with the former being more common.

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By © Raimond Spekking / CC BY-SA 4.0 (via Wikimedia Commons), CC BY-SA 4.0

A Sealed Unit

The reason why you can’t just simply open up a hard disk drive is because any contaminant, no matter how small, can cause irreversible data damage. The platters must be in a unique environment to be able to operate properly, as evident with many having a small hole on cover with the warning of “Do Not Cover!” nearby – that is there to equalize the air pressure which is another requirement of proper operation.

As soon as you expose the platters to the outside damage may have already occurred. Hard disk drives are assembled in rooms cleaner than surgery rooms and are sealed to not let anything in. Data recovery can only be done by specialist companies that are sufficiently equipped.


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NIUBI Partition Editor. 2019. Hard Disk and Hard Drive Physical Components. [online] Available at: <https://www.hdd-tool.com/hdd-basic/hard-disk-and-hard-drive-physical-components.html>.

optocrypto.com. 2018. What are the parts of a hard drive?. [online] Available at: <https://optocrypto.com/parts-hard-drive/>.

Pchardtech.blogspot.com. n.d. Parts of Hard Disk. [online] Available at: <https://pchardtech.blogspot.com/2010/03/parts-of-hard-disk.html>.

Torres, G., 2005. Anatomy of a Hard Disk Drive | Hardware Secrets. [online] Hardwaresecrets.com. Available at: <https://hardwaresecrets.com/anatomy-of-a-hard-disk-drive/>.

Woodford, C., 2020. How does a hard drive work?. [online] Explain That Stuff. Available at: <https://www.explainthatstuff.com/harddrive.html>.


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